CN108370267B

CN108370267B - Method for selecting broadband multimode optical fiber according to single wavelength characterization

Info

Publication number: CN108370267B
Application number: CN201580085182.XA
Authority: CN
Inventors: D·莫林; K·德容; M·比戈; P·斯拉德
Original assignee: Draka Comteq BV
Current assignee: Draka Comteq BV
Priority date: 2015-12-07
Filing date: 2015-12-07
Publication date: 2022-06-07
Anticipated expiration: 2035-12-07
Also published as: EP3387763C0; CN108370267A; WO2017098297A1; US10382124B2; EP3387763A1; EP3387763B1; US20180359025A1

Abstract

The invention relates to a method for selecting a broadband multimode optical fiber according to a single wavelength, comprising the following steps for each multimode optical fiber: obtaining (300) a first DMD graph using DMD measurements performed at a first single wavelength; obtaining (310) a first multimode fiber gauge parameter from the first DMD graph; and the method comprises for each optical fiber the steps of: obtaining (320) from said first DMD graph a curve representing radial offset delay as a function of radial offset value, referred to as ROD curve; applying (330) a linear fit to the ROD curve for at least two ranges of radial offset values; obtaining (340) an average radial offset delay slope, referred to as ROD slope, from a linear fit applied from across the range of radial offset values; selecting (350) a multimode optical fiber for which the first multimode optical fiber performance parameter meets a first predetermined specification standard and for which the calculated at least two ROD slopes meet a predetermined slope standard.

Description

Method for selecting broadband multimode optical fiber according to single wavelength characterization

Technical Field

The present invention relates to the field of optical fiber transmission, and more particularly to multimode optical fibers for use in relatively long distance and high bit rate systems.

More particularly, the present invention relates to a method of selecting a broadband multimode optical fiber based on a single wavelength characterization.

A broadband multimode optical fiber is herein understood to be a multimode optical fiber having an operating wavelength range of more than 20nm, in particular but not exclusively included between 850nm and 950 nm.

The invention is particularly applicable to broadband OM4 multimode fibers, but is not limited to broadband OM4 multimode fibers.

Background

Multimode optical fiber has evolved from the beginning of the optical communications industry until the current burst of ethernet traffic being experienced. High-speed multimode optical fibers implemented with VCSEL technology (e.g., OM4 optical fiber, which is a laser optimized high bandwidth 50 μm multimode optical fiber standardized by the international organization for standardization in the ISO/IEC11801 document and the TIA/EIA492AAAD standard), etc.) have proven to be the first medium of high data rate communication, providing a reliable and cost-effective solution of 10-100 Gbps. The combination of broadband (WB) multimode fibers with longer wavelength VCSELs for Coarse Wavelength Division Multiplexing (CWDM) is a considerable focus option for meeting the growth of future demands.

However, until now, the high mode bandwidth of OM4 fiber has only been achieved over a narrow wavelength range (typically 850nm +/-10 nm). The availability of broadband (WB) multimode fibers that meet OM4 performance requirements over a wider wavelength range is a challenge to overcome in next generation multimode systems.

OM4 fiber performance is typically defined by an Effective Mode Bandwidth (EMB) evaluation at a given single wavelength. For example, OM4 fiber should exhibit an EMB greater than 4700MHz-km at a wavelength of 850 nm. Achieving such high EMB values requires very accurate control of the refractive index profile of the multimode fiber. Conventional manufacturing processes to date have not been able to ensure such a high EMB, and it is often difficult to accurately predict the EMB value from refractive index profile measurements on the core rod or rod, especially where a high EMB (typically greater than 2000MHz-km) is expected, which means that the fiber refractive index profile is close to the optimum profile. Indeed, EMB was evaluated directly on the fiber.

To minimize modal dispersion, OM4 optical fibers typically include a core that shows a decreasing index of refraction from the center of the fiber to the point of engagement with the cladding. In general, the refractive index profile is given by a relationship known as "α profile", as follows:

wherein r is less than or equal to a

Wherein:

n₀is the refractive index on the optical axis of the fiber;

r is the distance from the optical axis;

a is the radius of the core of the optical fiber;

Δ is a dimensionless parameter representing the refractive index difference between the core and the cladding of the fiber; and

α is a dimensionless parameter representing the general shape of the refractive index profile.

The Alpha parameter (α) governing the shape of the graded-index core can be adjusted to maximize the modal bandwidth of OM4 multimode fiber at 850nm, which is a typical operating wavelength for high-speed data communications. As shown in Molin et al in the document "WideBand OM4Multi-Mode Fiber for Next-Generation 400Gbps Data Communications" (in ECOC 2014), a given value of the alpha parameter is typically selected to provide the best EMB.

The Effective Modal Bandwidth (EMB) is evaluated by measuring the Delay due to Modal Dispersion, known as the "Dispersion Modal Delay (DMD) graphical representation", by acronym. This consists in recording the impulse response of the multimode fiber for single mode injection for radial scanning of the core. This provides a DMD plot that is then post-processed to evaluate the minimum EMB that the fiber can provide at a given wavelength. The DMD measurement procedure is the subject of standardization (IEC60793-1-49 and FOTP-220) and is also specified in the Telecommunications industry Association's document number TIA-455-220-A. Each DMD metric or DMD value is expressed in units of picoseconds/meter (ps/m). This determines the delay between the fastest and slowest pulse through the fiber, taking into account the set of offset injections normalized by the fiber length. Each DMD metric or DMD value substantially evaluates modal dispersion. Low DMD values, i.e., low modal dispersion as measured by DMD, generally result in higher EMB.

Basically, the DMD graphical representation is obtained by injecting a light pulse of a given wavelength in the center of the fiber, the introduction of which has a radial offset to cover the entire core of the multimode fiber, and by measuring the pulse delay after a given fiber length L. The measurements are repeated so as to provide a plot of the modal dispersion of the multimode optical fiber under inspection at different radial offset values. These DMD measurements are then post-processed to determine the effective transfer function of the fiber, from which the value of EMB can be determined.

Currently, all multimode fiber manufacturers perform DMD measurements and EMB evaluations throughout their production at only a single wavelength, typically 850nm +/-2nm for OM4 certification and 850nm +/-10nm for OM3 certification.

With the advent of new multimode fiber applications requiring high EMB over a wide operating window, one of the main considerations of multimode fiber manufacturers is the ability to easily evaluate EMB over a wide range of wavelengths (e.g., between 850nm and 950 nm).

Using the classical measurement procedure described above (including a series of DMD measurements and EMB evaluations at a single wavelength) to evaluate the EMB of an optical fiber over a range of wavelengths (i.e. at multiple wavelengths) would require several measurement procedures at multiple wavelengths that are well dispersed over the wavelength range of interest. However, performing different individual DMD measurements at multiple wavelengths to authenticate the EMB of the fiber greatly results in increased measurement time and cost for measuring and producing broadband multimode fiber. Such a solution would obviously require the installation of several light sources each emitting at a different wavelength and several corresponding detectors, which would represent a complex and expensive operation.

Therefore, there remains a need for a simple and low cost method to identify multimode optical fibers that ensure high modal bandwidth over a broad wavelength spectrum from only a single wavelength characterization during production.

Document US 8,351,027 proposes to use metrics derivable from DMD measurements in combination with industry standard metrics such as effective modal bandwidth and DMD to obtain a more accurate prediction of multimode fibre channel link performance as measured by BER testing. This metric may be used to select or verify fiber performance at wavelengths close to the wavelength measured by the DMD.

The present invention, in at least one embodiment, provides a method that enables the EMB of a broadband multimode fiber over a relatively large spectral window with characterization limited to a single wavelength.

In at least another embodiment, the present invention provides a method for selecting a broadband multimode optical fiber from a collection of multimode optical fibers that is easy to implement and reduces multimode fiber measurement costs.

Disclosure of Invention

A particular embodiment of the invention proposes a method for selecting a broadband multimode optical fiber according to a single wavelength, comprising, for each multimode optical fiber, the steps of:

-using at a first single wavelength (λ)₁) Obtaining a first DMD plot comprising a plurality of traces each recorded with a different radial offset value r from 0 for the axis of the multimode optical fiber to a radial offset value r a, a being the core radius of the multimode optical fiber;

-obtaining at least one first multimode optical fiber specification parameter at the single wavelength from said first DMD graph;

and, for each multimode optical fiber, the method further comprises the steps of:

-obtaining from said first DMD map a curve representing the radial offset delay of said multimode optical fiber as a function of said radial offset value r, wherein this curve is called ROD curve, where 0 ≦ r ≦ a;

-applying a linear fit to the ROD curve for at least one range of radial offset values;

-obtaining a radial offset delay slope, referred to as ROD slope, from the linear fit applied for each range of radial offset values;

-selecting a multimode optical fiber for which the at least one first multimode optical fiber performance parameter meets a first predetermined specification standard and the calculated at least one ROD slope meets at least one predetermined slope standard.

The general principles of the present invention rely on the use of slope parameters calculated on a curve representing the radial offset delay of multimode optical fibers to derive broadband possible performance of these fibers from simple DMD characterization performed at a single wavelength.

ROD slope is defined as the slope given by a linear fit. The linear fit applied to the ROD curve approximates the ROD curve over a particular range of radial offsets, for example by an affine function.

According to a particular feature, for each predetermined slope criterion, a slope condition is verified for the calculated ROD slope and/or for a set of at least two calculated ROD slopes having the at least one calculated ROD slope, wherein the slope condition is defined by at least one threshold.

According to a particular feature, the at least one range of radial offset values comprises a first offset range (a) which is a radial offset range [ 0.75; 1.00] or the radial offset range [ 0.75; 1.00 ].

The first offset range corresponds to a refractive index profile characteristic of the optical fiber that is sensitive to changes in the refractive index profile at the core-cladding interface.

According to a particular feature, a first ROD slope is calculated for said first range of radial offset values, and wherein said step of selecting takes into account a first predetermined slope criterion defined as:

a first slope condition such that s_A<-3.0×10^-3，

Wherein s is_AA value of the first ROD slope in ps/m/μm.

Thus, the invention may be performed considering only one slope parameter in the selection step. Only the calculated slope s_ACompared to a given slope threshold below which the multimode fiber is selected. In other words, multimode fibers above the slope threshold are rejected.

According to a particular feature, the at least one range of radial offset values comprises:

-a second offset range (B) being a radial offset range [0.40 normalized for the core radius of the multimode optical fiber; 0.80] or the radial offset range [ 0.40; sub-range of 0.80], and

-a third offset range (C) being a radial offset range [0.0 normalized for the core radius of the multimode optical fiber; 0.25] or the radial offset range [ 0.0; 0.25 ].

The second shift range is sensitive to any refractive index profile deviation along the refractive index gradient, for example, a shift that would affect the alpha parameter value of the EMB provided by the fiber. The third shift range is sensitive to refractive index profile deviation around the center of the core (i.e., the optimum refractive index).

Thus, the step of applying a linear fit to the ROD curve is performed for one or more different ranges of radial offset values that each correspond to a characteristic portion of the refractive index profile of the optical fiber. The first offset range corresponds to a refractive index profile region near a core-cladding interface of the optical fiber; the third offset range corresponds to a refractive index profile region near the central axis of the optical fiber; the second shift range corresponds to an intermediate refractive index distribution region between the two regions.

According to a particular feature, a second ROD slope and a third ROD slope are calculated for a second range of radial offset values and a third range of radial offset values, respectively, the first predetermined slope criterion further comprising:

a second slope condition such that s_B<-1.4×s_C-2.0×10^-3，

Wherein s is_BAnd s_CValues of the second and third ROD slopes expressed in ps/m/μm.

The method according to the invention may further take into account a set of two slope parameters. Then, the second slope condition is defined by an affine function defining a multi-parameter threshold (as opposed to the first slope condition defined by a single parameter threshold (constant function)).

This first predetermined slope criterion corresponds to an "acceptance criterion" for multimode optical fibers having a possible Effective Mode Bandwidth (EMB), i.e. a possible broadband performance, equal to or higher than 2700MHz-km at a single wavelength of 950 nm.

According to a particular feature, said step of selecting takes into account a second predetermined slope criterion defined as:

a first slope condition such that s_A<-6×10^-3，

Wherein s is_AA value of the first ROD slope in ps/m/μm.

According to a particular feature, the second predetermined slope criterion (CROD2) further comprises:

a second slope condition such that s_B<-1.4×s_C-4.0×10^-3，

Wherein s is_BAnd s_CValues of the first, second, and third ROD slopes in ps/m/μm.

The second slope criterion corresponds to a narrow acceptance criterion for multimode optical fibers with potentially broadband performance. The second slope criterion may be combined with the first slope criterion to maximize the effectiveness of the measurement by preferentially measuring fibers that meet these more stringent criteria.

According to a particular feature, the length of the at least one range of radial offset values is equal to at least 0.04 of the normalized core radius.

A minimum tolerance of 1 micron was applied to a multimode fiber with a core radius of 25 microns, resulting in a normalized core radius of 0.04. This enables a sufficient number of points (i.e., sufficient DMD data) on the ROD curve to effectively apply a linear fit on the ROD curve for the range of offset values considered.

The "length" of the offset value range means a length included between a lower limit value and an upper limit value of the value range. For example, a first offset range [ 0.75; 1.00] has a length of 0.25(0.75 is the lower limit, 1.00 is the upper limit).

More specifically, the length of the at least one range of radial offset values is equal to at least 0.10 of the normalized core radius.

According to a particular feature, the core radius is 25 μm ± 1.25 μm, and:

-said first range (a) of radial offset values is comprised between 19 μ ι η and 23 μ ι η;

-said second range of radial offset values (B) is comprised between 10 μ ι η and 20 μ ι η;

-said third range (C) of radial offset values is comprised between 0 μ ι η and 6 μ ι η.

According to a particular feature, said at least one first multimode optical fibre specification parameter is an effective modal bandwidth at a single wavelength of 850nm, and at least one said first predetermined specification criterion is that said effective modal bandwidth is equal to or higher than 4700 MHz-km.

According to a particular feature, said at least one first multimode optical fiber specification parameter is an overfilled injection bandwidth at a single wavelength of 850nm, and at least one said first predetermined specification criterion is that the overfilled injection bandwidth is equal to or higher than 3500 MHz-km.

According to a particular feature, the method further comprises, for each multimode optical fiber selected, the steps of:

-obtaining a second DMD graph using dispersion mode delay measurements performed on the selected multimode optical fiber at a second single wavelength;

-obtaining at least one second multimode optical fiber gauge parameter (P2) at said second single wavelength from said second DMD graph;

and, the method further comprises a sub-selection step for further selecting a multimode optical fiber that meets a second predetermined specification standard.

According to a particular feature, said at least one second multimode optical fibre specification parameter is the effective modal bandwidth at a single wavelength of 950nm, and at least one said second predetermined specification criterion is that the effective modal bandwidth is equal to or higher than 2700 MHz-km.

In another embodiment, the invention relates to a computer program product comprising program code instructions for implementing the above-described method (in any of the different embodiments) when said program is executed on a computer or processor.

In another embodiment, the invention relates to a non-transitory computer-readable carrier medium storing a program which, when executed by a computer or processor, causes the computer or processor to perform the above-described method (in any of the different embodiments).

Drawings

Further features of embodiments of the invention will emerge from the following description, given by way of illustrative and non-exhaustive example, and from the accompanying drawings, in which:

figure 1 shows an example of an optical communication system implementing a multimode optical fibre;

fig. 2 provides a schematic illustration of the principle of the DMD measurement process;

figure 3 provides a flow chart of a particular embodiment of the method according to the invention;

fig. 4 depicts an example of a DMD graphical representation showing the calculation of the ROD curve obtained for a multimode optical fiber according to a particular embodiment of the invention;

fig. 5 graphically depicts the ROD curve obtained for the DMD trace according to fig. 4 as a function of the radial offset value r, according to a particular embodiment of the invention;

FIG. 6 illustrates an example of a calculation of the ROD slope of the ROD curve of FIG. 5 as a function of a selected range of radial offset values, in accordance with certain embodiments of the present invention;

figures 7 and 8 graphically depict the normalized distribution of ROD slope parameters for a set of multimode optical fibers (OM4 fibers) that meet OM4 requirements;

figure 9 shows a histogram representing the distribution of OM4 fiber set as a function of the effective mode bandwidth measured at a wavelength of 950 nm;

figures 10 and 11 graphically depict the normalized distribution of ROD slope parameters for the OM4 fiber set of figures 7 and 8 limited to fibers exhibiting an effective modal bandwidth at 950nm higher than 2700 MHz-km;

figures 12 and 13 graphically depict the probability distribution of an actual broadband OM4 fiber as a function of the slope parameter;

fig. 14 shows a simplified structure of a selection device according to a particular embodiment of the invention.

Detailed Description

Throughout the drawings herein, like elements and steps are designated by like reference numerals.

The method according to the invention described below is suitable for OM4 multimode optical fiber with a core diameter of 50 μm. Of course, the invention is not limited to this particular application and may be applied to any other type of multimode optical fiber.

The general principles of the present invention rely on the use of slope parameters calculated on a curve representing the radial offset delay of multimode optical fibers to derive broadband potential performance of these fibers from DMD characteristics limited to a single wavelength.

Fig. 1 shows an example of an optical communication system comprising a multimode optical fiber, which is the object of the present selection method. The multi-gigabit ethernet optical communication system comprises, in order, a driver 8 of the transmitter 1, a VCSEL source 9 of the transmitter 1, the injection line 2, a connector 3, a multimode fiber 4, a connector 3, the injection line 2, a PIN diode 6 of the receiver 5 and an amplifier 7 of the receiver 5. Digital signals at 10Gbps or 25Gbps are generated by a driver 8 for directly modulating a VCSEL source 9.

Fig. 2 shows a known principle of Differential Mode Delay (DMD) measurement. DMD measurement consists in injecting light pulses (ultrafast laser pulses) into a multimode optical fibre in sequence, wherein the light pulses have a given single mode wavelength (for example λ 1 ═ 850nm) and different radial offsets with respect to the centre of the fibre core between each successive pulse. The delay of each pulse passing through the fibre is then measured after a given length (L) of fibre. Each optical pulse is injected with a different radial offset value ("offset injection") r, from the central axis r of the fiber, 0 (i.e., the center of the fiber core) to r, a, where a is the core radius of the fiber. Each delay trace thus obtained corresponds to a given radial offset value.

More precisely, the optical reference pulse is emitted by the light source at a single wavelength (for example 850nm) and injected into the core 10 of a single-mode injection fiber having a core diameter of 5 μm. From the end of the single mode fiber, the optical reference pulse crosses the core of the multimode fiber (MMF)20 under test. The multimode optical fiber 20 typically has a core diameter of 50 μm. For each offset on the core (e.g., 0-25 microns in 1 micron increments), the output pulse is recorded by the high bandwidth optical receiver 30, giving the shape of the transmitted pulse, i.e., the DMD trace (also referred to as DMD measurement). The y-axis describes the radial offset r from the center of the fiber core in microns, and the x-axis describes time in picoseconds or nanoseconds. For example, the DMD measurement process starts with r-0 and ends with r-a. Typically, the difference in delay of the leading edge of the fastest pulse and the trailing edge of the slowest pulse (a typical threshold is 25% of maximum) across the fiber is used. The modal dispersion of the multimode fiber 30 is typically evaluated by calculating the difference between the fastest time and the slowest time taking into account a particular range of offset injection. These time delay differences are referred to as DMD values.

The example of the DMD graph 200 shown on fig. 2 shows a set of 24 recorded traces, each trace corresponding to a DMD measurement performed for a given radial offset value r from the center of the fiber optic core.

Fig. 3 shows a flow chart of a particular embodiment of a selection method according to the present invention.

A batch of multimode optical fibers was taken at the outlet of production. The standard radius of the multimode fiber is 25 μm (+ -1.25 μm). The aim of the method is to select, among a batch of optical fibres tested, an optical fibre which meets the OM4 standard specification and exhibits, with a high probability, an Effective Modal Bandwidth (EMB) equal to or higher than 2700MHz-km at a wavelength of 950 nm.

In step 300, at a wavelength of 850nm (λ) as set forth in the FOTP-220 standard₁) DMD measurements were made for each multimode fiber. At the end of this step, a DMD plot is obtained for each test fiber. Fig. 4 shows an example of a DMD graph obtained for a given fiber in a batch of fibers tested: the x-axis describes time in nanoseconds and the y-axis describes offset implantation in microns.

The following steps 310 to 340 are performed for each DMD graph obtained, but steps 310 to 340 are then described for a given DMD graph (this is to simplify the description of the present invention).

In step 310, the DMD data of the DMD graph obtained in the previous step is processed to obtain data representing 850nm (λ)₁) One or more OM4 fiber specification parameters of the fiber performance. For example, OM4 lightThe fiber specification parameter is EMB at a wavelength of 850nm (specification P1). The process of obtaining an EMB at 850nm from a DMD plot is well known to those skilled in the art. This process is described, for example, in document TIA-455-220-A (1/2013, FOTP-220) entitled "Differential model Delay Measurement of Multimode Fiber in the Time Domain".

Another OM4 fiber gauge parameter may be the OFL bandwidth at a wavelength of 850nm (gauge parameter P1'). The process of obtaining the OFL bandwidth at 850nm from a DMD plot is well known to those skilled in the art. This process is described, for example, in the document "calibrated Module Bandwidths of an OM4 Fiber and the therapeutic Challenges" (IWCS, Charlotte, N.C., 2009, page 24) of A.Sengutta.

In step 320, as shown in fig. 5, a curve representing radial offset delay (hereinafter referred to as ROD curve) as a function of radial offset value (r) is calculated from the DMD graph obtained in step 300. ROD is the average delay of the trace recorded during DMD measurement relative to the average delay of a reference trace, e.g., the trace corresponding to a center-offset implant (i.e., r-0 μm). ROD may be calculated as the center of gravity of the trace under consideration, as follows:

wherein s is_r(r) is in the time window [0, T]The trace recorded with an offset value r during the DMD measurement on, L is the length of multimode fiber tested.

The ROD curve is a function f (r) defined as:

f(r)＝ROD(r)-ROD(r_REF)

wherein r is_REFIs a reference offset value, where r_REF＝0μm。

Fig. 4 shows the radial offset delay for a given DMD graph. Each circle visualizes the average delay calculated for a given radial offset value. The example shown here corresponds to the average offset delay calculated for the DMD trace corresponding to a radial offset of 15 μm.

Fig. 5 shows the corresponding ROD curve f (r) obtained from the DMD graph of fig. 4 with the reference offset value set to 0 μm. The radial offset r is comprised between 0 and 25 μm (i.e. the fiber core radius). The function f (r) is expressed in ps/m.

In step 330, as shown in fig. 6, a linear fit is applied to three different ranges of radial offset values of the ROD curve obtained in the previous step:

the first offset range A (hereinafter referred to as the "outer offset range") corresponds to the region of the fiber refractive index profile near the core-cladding interface, e.g. 19. ltoreq. r.ltoreq.23 μm;

a third offset range C (hereinafter referred to as "inner offset range") corresponds to a region of the fiber refractive index profile near the central axis of the fiber, e.g., 0 ≦ r ≦ 6 μm;

the second shift range B (hereinafter referred to as the "intermediate shift range") corresponds to an intermediate fiber refractive index profile region located between the inner shift range and the outer shift range, e.g., 10 ≦ r ≦ 20 μm.

These three ranges of offset values are chosen because they each correspond to a characteristic portion of the refractive index profile of the multimode fiber that affects the evaluation of the effective modal bandwidth, namely:

the first offset range a is sensitive to the core-cladding interface, which means a refractive index profile of the first micrometer in the graded outer and surrounding cladding (which may be designed with grooves or rings, for example);

the second shift range B is sensitive to any deviation of the distribution along the refractive index gradient due to the value of the alpha parameter (the shape of the refractive index distribution is dominated by the alpha parameter), e.g. to a shift of the alpha parameter relative to the alpha parameter providing an optimal EMB at 850 nm;

the third shift range C is sensitive to refractive index profile deviations around the center of the core (i.e. deviations from the optimal refractive index at the center of the refractive index profile).

In this example, an offset range comprised between 24 and 25 μm is excluded, since it substantially corresponds to the noise portion caused by the measurement error. Of course, this particular offset range may be considered in the steps of the method without departing from the scope of the invention.

It should be noted that the second offset range may overlap one and/or the other of the first and third offset ranges. The expression "between the first and third offset ranges" as used herein does not exclude possible overlaps of different offset ranges.

More generally, however, the ROD curve f (r) may be divided into three ranges of radial offset values as follows:

-a first offset range a is a radial offset range [ 0.75-1.00 ] or a sub-range of the radial offset range [ 0.75-1.00 ] normalized (r/a) for a core radius of the multimode optical fiber;

-the second offset range B is a radial offset range [ 0.40-0.80 ] or a sub-range of the radial offset range [ 0.40-0.80 ] normalized (r/a) for the core radius of the multimode optical fiber;

-the third offset range C is a radial offset range [ 0.00-0.25 ] or a sub-range of the radial offset range [ 0.00-0.25 ] normalized (r/a) for the core radius of the multimode optical fiber.

The number of offset ranges used in this example is three. The invention is not limited to this example but may also be implemented with a larger or smaller number of offset ranges without departing from the scope of the invention. If the offset range is the outer offset range a, the method according to the invention can be implemented with only one such offset range (and thus only one slope condition as described below). The inventors have realized that a number of three offset ranges may give a satisfactory selectivity of the method according to an embodiment of the invention for a broadband multimode optical fiber that actually meets the required performance specifications.

The lengths of the first, second and third offset ranges (a, B, C) have range lengths of 0.25, 0.40 and 0.25 of the normalized radial offset, respectively. "length" means a length included between the lower and upper values of the range of values considered. More generally, the length between the lower and upper limits of each range of offset values according to the invention is at least 0.04 of the normalized core radius. In fact, imposing a minimum tolerance of 1 micron for a core radius of 25 microns gives a normalized core radius of 0.04. A minimum value of 0.04 ensures that there are a sufficient number of points on the ROD curve (i.e., sufficient DMD data) to effectively apply a linear fit on the ROD curve for the range of offset values considered.

In step 340, a radial offset delay slope (hereinafter referred to as ROD slope) is obtained from a linear fit applied for each radial offset value range set in the previous step.

ROD slope is the slope given by a linear fit for a given radial offset range. The linear fit applied to the ROD curve (f (r)) approximates the ROD curve over a particular inner, middle and outer offset range by an affine function, such as:

f(r)＝ROD Slope×r+Constant

thus, the linear fit gives two coefficients: slope 'ROD Slope' and offset 'Constant'.

The ROD slopes obtained for the inner, middle and outer offset ranges at the end of this step are referred to as "inner ROD slopes" (or "inner ROD slopes" in the figure) s, respectively_A"intermediate ROD slope" (or "intermediate ROD slope" in the figure) s_BAnd an "external ROD slope" (or "external ROD slope" in the figure) s_C。

These slope parameters calculated from DMD measurements at 850nm were used to evaluate fiber broadband performance at 950nm, in accordance with the present invention.

In step 350, after the previous steps 300 to 340 have been performed for each multimode optical fiber in the batch of optical fibers, the multimode optical fiber that meets the following criteria is selected:

-a multimode optical fiber satisfying a predetermined specification standard CP1 for the OM4 optical fiber specification parameter P1 obtained in step 310, and

multimode optical fiber for which the internal ROD slope, the intermediate ROD slope and the external ROD slope calculated in step 340 satisfy at least one predetermined slope criterion (hereinafter referred to as CROD1 or CROD 2).

For example, ifEMB (P1) at 850nm calculated in step 310 for this fiber is equal to or higher than 4700MHz-km (CP1), and if the internal ROD slope(s) calculated in step 340_A) Intermediate ROD slope(s)_B) And outer ROD slope(s)_C) The fiber is selected in accordance with a first predetermined slope criterion (hereinafter CROD 1).

In order to comply with the first slope criterion CROD1 according to the invention, a slope parameter s_A、s_BAnd s_CThe following slope conditions must be passed:

-a first slope condition such that s_A<-3.0×10^-3ps/m/μm, and

-a second slope condition such that s_B<-1.4×s_C-2.0×10^-3ps/m/μm。

If these slope conditions are met, this means that the fiber under consideration meets the first slope criterion CROD 1. This first slope criterion corresponds to an "acceptance criterion" for multimode fibers with a possible Effective Modal Bandwidth (EMB) equal to or higher than 2700MHz-km at a single wavelength of 950 nm.

Thus, the specification standard CP1 and slope standard CROD1 are intended to select fibers that exhibit OM4 performance at 850nm and EMB above 2700MHz-km at 950 nm.

The first slope condition for CROD1 is represented, for example, in FIG. 12 by slope threshold S_TH11Shown. The slope condition is defined by being equal to-3.0 × 10^-3A constant function definition of ps/m/μm. The second slope condition for CROD1 is represented, for example, by threshold S in FIG. 13_TH21Shown. The slope condition consists of two slope parameters s_BAnd s_CIs defined (in other words, the slope condition is defined by a multi-parameter threshold). The present invention is not limited to the use of constant functions and affine functions; can use the utilization s_A、s_BAnd s_CMore complex rules of (2).

A second predetermined slope criterion (CROD2) may also be tested for each fiber independently or as a further of the first slope criterion. The second acceptance criterion is more stringent than the first slope criterion (CROD 1).

To is coming toComplying with the second slope criterion CROD2 according to the invention, slope parameter s_A、s_BAnd s_CThe following slope conditions must be passed:

-a first slope condition such that s_A<-6×10^-3ps/m/μm; and

-a second slope condition such that s_B<-1.4×s_C-4.0×10^-3ps/m/μm。

If these slope conditions are verified, the second slope criterion CROD2 is met for the relevant fiber, which means that the relevant fiber may exhibit an EMB higher than 2700MHz-km at 950nm with a higher probability (the probability of actually meeting the OM4 specification and providing an EMB equal to or higher than 2700MHz-km at 950nm is greater than 90%).

The second slope criterion CROD2 has a narrower condition for slope values than the first slope criterion CROD 1.

The first slope condition for CROD2 is represented, for example, in FIG. 12 by slope threshold S_TH21Shown. The slope condition is defined by being equal to-6.0 × 10^-3A constant function definition of ps/m/μm. The second slope condition for CROD2 is represented, for example, by threshold S in FIG. 13_TH22Shown. The slope condition consists of two slope parameters s_BAnd s_CIs defined (in other words, the slope condition is defined by a multi-parameter threshold). The present invention is not limited to the use of constant functions and affine functions; can use the utilization s_A、s_BAnd s_CMore complex rules of (2).

The idea of this particular embodiment is to pre-select the optical fibers that are most likely to meet the requirements at 950nm in order to prioritize or limit the actual DMD measurement made on these fibers at 950 nm.

It should be noted that each slope condition discussed in this example is defined by a single slope threshold (first slope condition) or a multiple slope threshold (second slope condition). Of course, the slope condition may be defined by two thresholds (e.g., a minimum threshold and a maximum threshold) without departing from the scope of the present invention. Furthermore, it is important to note that the slope condition can be employed as a function of the wavelength range to be covered and the wavelength used for the DMD measurement.

To evaluate the efficiency of the above criteria, actual DMD measurements at 850 and 950nm have been made on a set of fibers that meet the OM4 requirements (hereinafter "OM 4 fibers"). Fig. 7 and 8 graphically depict normalized distributions of slope parameters (inner ROD slope, intermediate ROD slope, and outer ROD slope) for OM4 fiber group, thus meeting specification standard CP 1. The left y-axis describes the value of the intermediate ROD slope, and the x-axis describes the value of the inner ROD slope (fig. 7) or the outer ROD slope (fig. 8). Note that x 10 is in the x-axis and y-axis^-3The coefficient of (a). DMD measurements of this multimode fiber set have been made at a wavelength of 850 nm. The internal ROD slope, the intermediate ROD slope, and the external ROD slope have been calculated using the above-described offset ranges A (0. ltoreq. r.ltoreq.6 μm), B (10. ltoreq. r.ltoreq.20 μm), and C (19. ltoreq. r.ltoreq.23 μm).

Fig. 10 and 11 graphically depict normalized distributions of slope parameters (inner ROD slope, intermediate ROD slope, and outer ROD slope) for OM4 fiber set limited to fibers that also exhibit EMB above 2700MHz-km at 950 nm. The left y-axis depicts the middle ROD slope and the x-axis depicts either the inner ROD slope (fig. 10) or the outer ROD slope (fig. 11). Note that x 10 is in the x-axis and y-axis^-3The coefficient of (a).

The maximum occurrence of the normalized distributions of fig. 7, 8, 10 and 11 is arbitrarily set to 100.

Fig. 9 reports the EMB distribution at 950nm for this OM4 fiber set. The OM4 fiber to be selected is one exhibiting an EMB equal to or greater than 2700MHz-km at 950nm (i.e., an EMB located to the right of the dotted line on fig. 9).

Figures 12 and 13 graphically depict the probability distribution of an effectively broadband OM4 fiber as a function of the inner ROD slope, the intermediate ROD slope, and the outer ROD slope. The two graphs are each within [ inner ROD slope; intermediate ROD slope]Space (fig. 12) and [ intermediate ROD slope intermediate; external ROD slope]The space (fig. 13) is established by the ratio of the number of broadband fibers (fig. 10 and 11) to the number of OM4 fibers (fig. 7 and 8). The left y-axis describes the value of the intermediate ROD slope, and the x-axis describes the value of the inner ROD slope (fig. 12) or the outer ROD slope (fig. 13). Note that the x-axis and y-axis have×10^-3The coefficient of (a). The values calculated for a given pair of mid-inner ROD slopes (fig. 12) or a given pair of mid-outer ROD slopes (fig. 13) correspond to the probability (expressed in percent of gray scale on the right y-axis) that the tested fiber is broadband OM4 (i.e., a fiber that conforms to the OM4 specification standard (CP1) and has an EMB higher than 2700MHz-km at 950 nm). These graphs highlight acceptable and unacceptable regions with respect to slope parameters characterized by the percentage of fiber that must exhibit broadband OM4 performance as defined in the present invention.

Thus, the graphs of fig. 12 and 13 enable the probability of OM4 fiber being actually broadband to be evaluated from only the inner ROD slope, the intermediate ROD slope, and the outer ROD slope measured at 850 nm. These patterns can then be used to decide whether or not to actually make measurements at 950 nm.

Note that the distribution of broadband OM4 fiber (meaning OM4 fiber exhibiting EMB >2700MHz-km at 950 nm) is within the [ internal ROD slope; intermediate ROD slope ] space is more limited to the left, and between [ intermediate ROD slope; outer ROD slope ] space is more limited to the lower left. This demonstrates that the selection over the three shift ranges A, B and C described above helps detect a potentially broadband OM4 fiber from the DMD characteristic wavelength at 850 nm.

Thus, for example, it can be seen that there are a number such as [ s ]_A<-6×10^-3ps/m/μm]And [ s ]_B<-1.4×s_C-4×10^-3ps/m/μm]Multimode fibers of equal slope values exhibit a probability of greater than 90% of actually meeting the OM4 specification and providing an EMB at 950nm equal to or higher than 2700 MHz-km. Thus, the optical fiber of the batch of optical fibers may be selected that exhibits a probability greater than 90%. Of course, a threshold of 90% is an example, and other thresholds may be set for implementation of the method depending on the desired trade-off.

For example, it can also be seen that there are, for example, [ s ]_A>-3×10^-3ps/m/μm)]And [ s ]_B>-1.4×s_C-2×10^-3ps/m/μm]Multimode fibers of equal slope values have a probability of less than 6% of having an EMB at 950nm equal to or higher than 2700 MHz-km. Can be aimed at those lights that do not meet the performance probability criteriaThe fiber saves the cost of making a measurement at 950 nm.

It has been shown that due to the method of the invention about 28% of the measurements at 950nm can be avoided.

The present invention is not limited to this particular embodiment and may also be implemented with a greater or lesser number of slope conditions without departing from the scope of the invention. For example, if the slope condition is with respect to an external ROD slope s_AThen the method according to the invention can be simply implemented with only this one slope condition. In this case, the probability of actually meeting the OM4 specification and providing an EMB at 950nm equal to or higher than 2700MHz-km may be less than if two slope conditions were used in the method.

Moreover, another predetermined specification standard CP1 'for OM4 fiber specification parameter P1' obtained in step 310 can also be verified in the fiber selection step: for example, if the OFL bandwidth (P1') obtained at 850nm in step 310 for this optical fiber is equal to or higher than 3500MHz-km (CP1'), this optical fiber is selected.

In a particular embodiment, the method may then include the following steps (not shown in fig. 3) for each multimode optical fiber selected in the previous step 350:

use of a single wavelength (λ) at 950nm₂) Performing a Dispersion Mode Delay (DMD) measurement on the selected multimode fiber to obtain a second DMD graph;

-obtaining a second DMD diagram representing a wavelength (λ) at 950nm₂) One or more OM4 fiber specification parameters P2 of optical fiber performance;

-further selecting, for the gauge parameter P2, a multimode optical fiber complying with a second predetermined gauge standard CP 2: for example, EMB at 950nm (P2) must be equal to or higher than 2700MHz-km (CP 2).

Fig. 14 shows a simplified structure of a selection means 60 according to a particular embodiment of the invention, wherein the selection means 60 for example performs the selection method shown in fig. 3.

The apparatus 60 includes a non-volatile memory 61 (e.g., a Read Only Memory (ROM) or hard disk), a volatile memory 63 (e.g., a random access memory RAM), and a processor 62. The non-volatile memory 61 is a non-transitory computer readable carrier medium. The non-volatile memory 61 stores executable program code instructions executed by the processor 62 to enable the selection method described above in connection with fig. 3.

At initialization, the above program code instructions are transferred from the non-volatile memory 61 to the volatile memory 63 for execution by the processor 62. The volatile memory 63 also includes registers for storing variables and parameters required for this execution.

Device 60 receives as input DMD measurement data 64 for each multimode optical fiber tested. Apparatus 60 produces as outputs for each multimode optical fiber tested:

probability level (e.g. percentage) of the optical fiber meeting the OM4 specification and broadband performance standard mentioned above, and/or

-an indication to select or not select this fiber for further EMB actual measurement.

All the steps of the above selection method can be equally well implemented by:

by executing a set of program code instructions executed by a reprogrammable computer, such as a PC-type device, a DSP (digital signal processor) or a microcontroller. The program code instructions may be stored on a removable or non-removable non-transitory computer readable carrier medium (e.g., a floppy disk, a CD-ROM, or a DVD-ROM);

implementation by means of a special-purpose machine or component, such as an FPGA (field programmable gate array), an ASIC (application-specific integrated circuit) or any special-purpose hardware component.

In other words, the invention is not limited to a purely software-based implementation in the form of computer program instructions, but may also be implemented in any form of hardware or a combination of hardware and software parts.

Although the present invention has been described with reference to examples of predetermined slope criteria having particular conditions, workers skilled in the art will recognize that changes may be made in form and detail without departing from the scope of the invention and/or the appended claims. Indeed, the above-mentioned specific conditions (e.g. threshold values) may be applied to derive the possible performance of the multimode optical fiber for wavelength ranges other than 850-.

Claims

1. A method for selecting a broadband multimode optical fiber according to a single wavelength, the method comprising, for each multimode optical fiber, the steps of:

using at a first single wavelength λ₁Obtaining a first DMD graph (300) comprising a plurality of traces each recorded with a different radial offset value r from 0 at the multimode fiber axis to a radial offset value r at a, a being the multimode fiber core radius;

-obtaining (310) at least one first multimode fiber gauge parameter P1 at said first single wavelength from said first DMD graph;

characterized in that, for each multimode optical fiber, the method further comprises the steps of:

-obtaining, from said first DMD map, a curve (320) representing the radial offset delay of said multimode optical fiber as a function of said radial offset value r, where this curve is called ROD curve, 0 ≦ r ≦ a, where ROD is the average delay of the traces recorded during DMD measurement with respect to the reference trace, said ROD being calculated as the center of gravity of the trace considered, as follows:

wherein s is_r(r) is in the time window [0, T]The trace recorded during DMD measurement at an offset value r, L being the length of multimode fiber tested, said ROD curve being defined as a function f (r) as follows: (r) ROD (r)_REF)，r_REFIs a reference offset value, where r_REF＝0μm；

-applying a linear fit (330) to the ROD curve for at least one range of radial offset values;

-obtaining a radial offset delay slope, referred to as ROD slope, from the linear fit applied for each range of radial offset values (340);

the method further comprises a selecting step (350) for selecting a multimode optical fiber complying with a first predetermined specification standard CP1 for said at least one first multimode optical fiber specification parameter P1 and the calculated at least one ROD slope complying with at least one predetermined slope standard.

2. The method according to claim 1, wherein for each predetermined slope criterion a slope condition is verified for the calculated ROD slope and/or a set of at least two calculated ROD slopes of the at least one calculated ROD slope, wherein the slope condition is defined by at least one threshold.

3. The method according to claim 1 or 2, wherein the at least one range of radial offset values comprises a first offset range a, the first offset range being a radial offset range [ 0.75; 1.00] or the radial offset range [ 0.75; 1.00 ].

4. The method of claim 3, wherein a first ROD slope s is calculated for the first offset range A_AWherein said selecting step considers a first predetermined slope criterion CROD1 defined as follows:

a first slope condition such that s_A<-3.0×10^-3，

Wherein s is_AThe value of the first ROD slope in ps/m/μm.

5. The method of claim 4, wherein the at least one range of radial offset values further comprises:

-a second offset range B being a radial offset range [ 0.40; 0.80] or the radial offset range [ 0.40; sub-range of 0.80], and

-a third offset range C, the third offset range being a radial offset range [0.0 normalized for the core radius of the multimode optical fiber; 0.25] or the radial offset range [ 0.0; 0.25 ].

6. The method of claim 5, wherein a second ROD slope s is calculated for a second offset range B and a third offset range C, respectively_BAnd a third ROD slope s_CThe first predetermined slope criterion CROD1 further includes:

a second slope condition such that s_B<-1.4×s_C-2.0×10^-3，

Wherein s is_BAnd s_CValues of the second and third ROD slopes in ps/m/μm.

7. The method according to claim 6, wherein said selecting step considers a second predetermined slope criterion CROD2 defined as:

a first slope condition such that s_A<-6×10^-3，

Wherein s is_AThe value of the first ROD slope in ps/m/μm.

8. The method of claim 7, wherein the second predetermined slope criterion CROD2 further comprises:

a second slope condition such that s_B<-1.4×s_C-4.0×10^-3，

Wherein s is_BAnd s_CValues of the second and third ROD slopes expressed in ps/m/μm, respectively.

9. The method of claim 3, wherein the at least one range of radial offset values has a length equal to at least 0.04 of the normalized core radius.

10. The method of claim 3, wherein the at least one range of radial offset values has a length equal to at least 0.10 of the normalized core radius.

11. The method of claim 5, wherein the core radius is 25 μ ι η ± 1.25 μ ι η, and:

-said first offset range a is comprised between 19 μ ι η and 23 μ ι η;

-said second offset range B is comprised between 10 μ ι η and 20 μ ι η;

-said third offset range C is comprised between 0 μm and 6 μm.

12. The method according to claim 1 or 2, wherein the at least one first multimode fiber gauge parameter P1 is an effective mode bandwidth at a single wavelength of 850nm, EMB, and the at least one first predetermined gauge standard CP1 is that the effective mode bandwidth is equal to or higher than 4700 MHz-km.

13. The method of claim 1 or 2, wherein the at least one first multimode fiber gauge parameter P1 is an overfill injection bandwidth (OFL bandwidth) at a single wavelength of 850nm, and at least one of the first predetermined gauge CP1 is that the overfill injection bandwidth is equal to or higher than 3500 MHz-km.

14. The method according to claim 1 or 2, wherein for each selected multimode optical fiber, further comprising the steps of:

using at a second single wavelength λ₂Obtaining a second DMD graph from measurements of the dispersion mode delay, or DMD, performed on the selected multimode fiber;

-obtaining at a second single wavelength λ, from said second DMD graph₂At least one second multimode fiber gauge parameter P2;

the method also includes a sub-selection step for further selecting a multimode optical fiber that meets a second predetermined specification standard CP 2.

15. The method of claim 14, wherein the at least one second multimode fiber gauge parameter P2 is an effective mode bandwidth at a single wavelength of 950nm (EMB), and at least one of the second predetermined gauge standards CP2 is that the effective mode bandwidth is equal to or higher than 2700 MHz-km.

16. A non-transitory computer-readable carrier medium storing a computer program product, characterized in that the computer program product comprises program code instructions for implementing the method according to any one of claims 1 to 15 when the program is executed on a computer or processor.